Codoped porous carbon nanofibres as a potassium metal host for nonaqueous K-ion batteries

Potassium metal is an appealing alternative to lithium as an alkali metal anode for future electrochemical energy storage systems. However, the use of potassium metal is hindered by the growth of unfavourable deposition (e.g., dendrites) and volume changes upon cycling. To circumvent these issues, we propose the synthesis and application of nitrogen and zinc codoped porous carbon nanofibres that act as potassium metal hosts. This carbonaceous porous material enables rapid potassium infusion (e.g., < 1 s cm−2) with a high potassium content (e.g., 97 wt. %) and low potassium nucleation overpotential (e.g., 15 mV at 0.5 mA cm−2). Experimental and theoretical measurements and analyses demonstrate that the carbon nanofibres induce uniform potassium deposition within its porous network and facilitate a dendrite-free morphology during asymmetric and symmetric cell cycling. Interestingly, when the potassium-infused carbon material is tested as an active negative electrode material in combination with a sulfur-based positive electrode and a nonaqueous electrolyte solution in the coin cell configuration, an average discharge voltage of approximately 1.6 V and a discharge capacity of approximately 470 mA h g−1 after 600 cycles at 500 mA g−1 and 25 °C are achieved.

iv) The MSCNF material is very porous (meso and micro) and has also considerable amount of voids between the fibres. While this is beneficial for depositing a large amount of K in the structure it will also require very large amount of electrolyte. The authors should comment on this fact in relation to a final energy density of the cell. v) In cross sectional image S30 (should actually be in main text for clarity) it looks like a considerable amount of K is deposited on top of the MSCNF host (a similar impression is given by figure 3f as well). The authors should clarify this, how much is in the host and how much is on top of the host material? In fact in figure 3b it also looks looke some deposition occurs on top, but hits figure is very difficult to interpret.
vi) If the majority of the K-metal is deposited in the host structure one would in fact expect a certain volume change of the electrode, since this kind of fibre structure is not very rigid. This should be confirmed by cross -sectional images with SEM or other methods. vii) A coulombic efficiency of 97% is stated. But for long cycle life this is not really enough, one would very fast deplete the cell of K at this CE level. For the experiments reported here a huge excess of K is present in the cell. The authors should comment on how to reach values need for real applications (>99.99%) where there is no excess K in the cell.
viii) A minor point is that how the specific capacity was calculated should be discussed in main text for clarity. Now it is discussed in SI.

Dear Authors,
The manuscript is well presented, complete and contains high-quality science that will be of significance to the field of batteries. The novelty of the paper is quite high despite many small typos found along with the manuscript. The authors carefully tailored the synthesis, performed many characterizations of the MSCNFs anode materials and, finally, they evaluated the electrochemical performance in a full Sulfur cells. I found the choice of testing the material in a sulfur cell very safe and low innovative but the material has high potassiophilicity, shows great performance and, therefore, the value of the manuscript remains quite high. I recommend the publication in nature communication after addressing the following comments.
-Please, find enclosed the attached file, which contains all the typos check in the manuscript and fix them.
- Figure 3 c and d. Define this measure In-situ XRD is completely wrong. The measure is an operando XRD analysis. I kindly ask the authors to describe the electrode preparation and the operando cell setup used for the reported measurement.
- Figure 4. Please, add in the caption that the spectra are XPS spectra. It might be not obvious to some young readers.
-Etching of the MSCNFs. Could, the authors elaborate, please, on the choice of HCl? And add this explanation in the preparation section.
-As I said, I found too safe the choice of coupling the anode with a sulfur-base cathode. The manuscript shows that the anode works for conversion materials. However, does it work with intercalation type cathodes? The title of the manuscript does not mention any sulfur system, so I suppose that these new materials are enabling the use of potassium metal anodes in different kinds of systems.

Reviewer #1 (Remarks to the Author):
The manuscript reports on a new host strategy for stable K-metal deposition. Overall the study shows very interesting results, with a very high capacity of the new material bearing promise for future real applications. The new findings and insights presented in the manuscript are of high interest and thus could very well be considered for publication, however only after addressing several issues which are need to be clarified to understand the full impact of this work.

Response:
We appreciate your positive comments and recommendation on this article. We are grateful for your support on this work. And we also thank you for your specific comments below. In general, we sincerely hope that the supplemented data added and the further explanations given will eliminate your concerns.
i) In the introduction the authors should better introduce state of the art to set their work in perspective. The motivation for the choice of Zn as a dopant for the MSCNF, also by relating to previous work in literature.

Response:
To give a detailed introduction, the following information is provided: Typically, metal-based hosts such as Cu and Al are a class of potential hosts for stable and dendrite-free composite K anode considering their commercial availability.
These metal-based hosts all demonstrate improved CE and suppressed overpotential, proving the validity of introducing potassiophilic sites to hosts. However, the intrinsic high density and relatively low space utilization hinder the application of metal hosts.
In contrast, carbon-based hosts are more favorable for K accommodation mainly because of their lightweight and electrochemical stability, which can realize higher volumetric or gravimetric energy density. HNCP/G (hollow N-doped C polyhedrons/graphene composite) and aligned carbon nanotube membrane (ACM), as pure carbon hosts, can realize prolonged symmetric cells cycling time (> 100 h) and decreased overpotential (< 0.1 V) (ACS Nano 2019, 13, 9306−9314;Adv. Energy Mater. 2019, 9, 1901427). In these cases, robust and highly conductive carbon-based hosts successfully evidence their potential for K accommodation. Consequently, an incorporation of carbon-based materials with other kinds of potassiophilic species becomes popular to deliver enhanced K plating/stripping electrochemistry. For example, PM/NiO (puffed millet/NiO) gives an enhanced K deposition volume and smaller voltage hysteresis due to the collaboration of PM host and potassiophilic NiO nanoparticles (Nano Energy 2019, 62, 367-375), PCNF@SnO2 (SnO2-coated conductive porous carbon nanofiber) realizes high K uptake (87%@15 mg cm −2 ) and uniform K nucleation taking advantage of its void-rich carbon nanofibers and potassiophilic SnO2 coating (J. Mater. Chem. A 2020, 8, 5671-5678), DN-MXene/CNT (defect-rich and nitrogen-containing MXene/carbon nanotube) achieves high CE (98.6%) and prolonged cycle life on account of its titanium defects and interconnected carbon scaffolds (Adv. Mater. 2020, 32, 1906739). Beyond the above metal compounds, metal nanoparticles might also draw attention for their potential of high K affinity and ease of synthesis. Previously, Au (Nat. Energy 2016, 1, 16010;Adv. Energy Mater. 2018, 8, 1802352), Ag (Adv. Mater. 2017, 29, 1702714;Sci. Adv. 2021, 7, eabg3626) and Zn (Adv. Energy Mater. 2018, 8, 1703505;Angew. Chem. Int. Ed. 2021, 60, 8515-8520) nanoparticles have been proven to be advantageous to lead a heterogeneous seeded growth of Li on carbon hosts. These results suggest that an introduction of metal nanoparticles (Pb, Sb, Sn and Zn) good at forming a K alloy would be an efficient strategy to improve the potassiophilicity of carbon hosts.
Per request, the corresponding introduction has been given in the revised manuscript (page 4).

ii) Role of formation cycles? Are the formation cycles performed before the electrochemical tests (in e.g. fig 2) or is for instance the data shown in fig 2a part of the formation cycle?
Response: "Formation cycles" is usually applied prior to electrochemical testing of metal anodes, it is a standard practice to have the working electrode current collectors undergo several discharge/charge cycles aiming to stabilize the SEI before plating and remove any electrochemically unstable residual impurities (Adv. Mater. 2020, 32, 2002908;Adv. Mater. 2020, 32, 1906735). In our study, formation cycles are performed for electrochemical tests relating to half cells such as CE tests (Figure 2c We have included the above information in the revised manuscript (page 9; method section).

iii) In the data shown in figure 2a a very small overpotential is found for the MSCNF.
However, an additional difference between the nano-fiber materials and Cu is the delay in the nucleation. Could the authors explain this? Is this related to SEI formation and if so, why does it seem to here consume much more capacity than for Cu? In relation to SEI formation it would be valuable to actually see the voltage profile and the nucleation potential in subsequent cycles.

Response:
From previous reports, it is a common phenomenon for carbon-based or highly porous hosts to experience a delay in the nucleation process compared to nonporous metal substrates such as Cu foils, which can be ascribed to SEI formation that consumes more electrolytes on the substrates with large surface area (Nat. Energy 2016, 1, 16010;Adv. Mater. 2020, 32, 1906739;Adv. Energy Mater. 2020, 2002654;Energy Technol. 2021, 9, 2000700).
By comparing the subsequent voltage profiles of the MSCNF||Cu cell with its 1st cycle ( Figure R3), it can be observed that the trend of delay gradually weakens and completely disappears at the 5th cycle, in consistent with its relatively low coulombic efficiency in the initial cycles. Similar trends exist in K-graphite batteries using carbonate electrolytes, where the graphite electrode exhibits a short voltage plateau near 0.7 V on the first potassiation curve and disappears in the following cycles (Adv. Funct. Mater. 2016, 26, 8103-8110;Energy Storage Mater. 2020, 24, 319-328).
Besides, the nucleation overpotentials in the subsequent cycles stay at ~20 mV, which show neglect variation. Consequently, it can be speculated that the delay of K plating on MSCNFs in the nucleation process is mainly due to the SEI formation at the 1st discharge, and the porous feature of MSCNFs make them consume much more capacity than Cu foils during this process.
We have included the corresponding discussion and Figure   iv) The MSCNF material is very porous (meso and micro) and has also considerable amount of voids between the fibres. While this is beneficial for depositing a large amount of K in the structure it will also require very large amount of electrolyte. The authors should comment on this fact in relation to a final energy density of the cell.

Response:
This is a very good perspective that clearly points out the critical issue that metal anodes based on a porous-host design currently confront for practical applications. It is also a problem for MSCNF. Actually, in this work, to demonstrate the advantage of our material design for outstanding cycling performance under a high K loading, we used composite MSCNF-K anodes fully infused with K metal to fabricate full cells.
As a result, the composite anode avoids the exposure of high surface area of MSCNF, which does not consume a large amount of electrolyte.
However, in pursuit of higher energy density, it indeed becomes a problem when we use a MSCNF host for an anode-free K battery or a K-metal battery with low N/P ratio (<2). In these systems, a MSCNF host would require excessive amounts of electrolyte and therefore decrease the energy density of the cell. In fact, there are several potential strategies that may reduce the requirement of electrolyte in a metal battery based on a porous anode host: (1) Decreasing the thickness of a porous host (<50 μm) via optimized material preparation method. For example, a proper electrospinning method is capable of fabricating a freestanding film as thin as 10 μm Mater. 2022, 34, 2109767). Hence, how to increase the energy density of a K metal battery based on porous anode hosts still remain a challenge for the practical application of K metal batteries, associative research is ongoing in our lab.
Per request, we have included the above discussion in the revised manuscript (page 19).

v) In cross sectional image S30 (should actually be in main text for clarity) it looks like a considerable amount of K is deposited on top of the MSCNF host (a similar
impression is given by figure 3f as well). The authors should clarify this, how much is in the host and how much is on top of the host material? In fact in figure 3b it also looks looke some deposition occurs on top, but hits figure is very difficult to interpret.

Response:
We are sorry for our uninformative presentation of morphology images correlating with K deposition, which might cause misleading information. Actually, there is a homogeneous K deposition taking place across the whole MSCNF host. To address the reviewer's concerns, we have now added some instructive marks and labels in the morphology images obtained from SEM and in-situ optical microscopy to better demonstrate the details. Specifically, from the cross-sectional images ( Figure   R4a), the MSCNF host retains its fibrous feature after a shallow discharge at 0.5 mA h cm −2 , and large amounts of voids can be observed within the host. This result gives the fact that the host is still in the initial state of K plating where the majority of the K metal deposits inside the pores and the surface of the nanofibers. Besides, some aggregated K particles form and uniformly distribute within the whole MSCNF host.
On the contrary, there is no recognizable aggregates on the top of the host. Additionally, cross-sectional EDS elemental mappings of the plated MSCNFs at different discharged states has been given in Figure R5 and R6, which confirm the homogeneous distribution of K in the deposited host, especially for the bottom of MSCNFs near Cu foils.  As for the in-situ optical microscopy observation, it has to be pointed out that the MSCNF host shows a mossy-like morphology on the top throughout the whole plating process because of its loosely packed carbon nanofibers near the surface ( Figure R7b, highlighted with red arrows). Due to a lack of space confinement from the vertical direction, the nanofibers on the top move freely under the driving force of electric field and ion flux, leading to an optical illusion that some deposition occurs on the top. In fact, it is difficult for us to investigate the precise position of K plating from this kind of optical observation, the result shown here only make us conclude that MSCNFs can effectively induce a homogeneous K deposition without the formation of rod-like K dendrites and therefore achieve a dendrite-free K anode.

Figure R7
In-situ optical microscopy observation of K deposition on (a) Cu foil and (b) MSCNFs at a current density of 6 mA cm −2 .
We have now included the above discussion and Figure R4

Response:
Theoretically, the relationship between deposition capacity and the thickness of K plated on a flat substrate can be described via the following equation: where T is the thickness, Q is the areal capacity, M is the molar mass, NA is the Avogadro constant, e is the electron charge, ρ is the density. Accordingly, it can be calculated that a K deposition capacity of 0.5, 3 and 5 mA h cm −2 corresponds to a thickness of 8.5, 50.7 and 84.4 μm, respectively.

The cross-sectional images of K deposited on Cu foils show a thickness of ~15
and ~84 μm at the discharge capacity of 0.5 and 3 mA h cm −2 , respectively (Supplementary Figure 31). In contrast, the deposited MSCNF hosts show a thickness of ~59 and ~58 μm at the discharge capacity of 0.5 and 3 mA h cm −2 , respectively ( Figure R4). Therefore, we can conclude that: (1) due to a lack of effective guidance and space confinement, the K metal deposited on Cu foils tends to form a loose and porous film, whose thickness is much higher than the theoretical value; (2) MSCNFs provide distinct regulation for K deposition that guarantees a dense and flat metal composite film. What's more, the neglectable change on the thickness of the deposited MSCNF at different states verifies our speculation that the K plating process can be divided into three phases ( Figure R8): Phase I, K metal starts to nucleate inside the pores of MSCNFs, finally fills up the pores and fully covers the carbon nanofibers (<0.5 mA h cm −2 ); Phase II, the metal cover on the nanofibers continues to grow and fills up the voids between the carbon nanofibers, forming a dense composite film (<3 mA h cm −2 ); Phase III, an extra amount of K metal continues to deposit on the MSCNF-K and thicken the film, leading to a volume change that follows the theoretical value.

Figure R8
Schematic illustration of K plating on MSCNF.
To confirm our speculation, the thickness of the MSCNF electrode discharged at 5 mA h cm −2 was measured. By subtracting the thickness of Cu foil (10 μm), the obtained electrode has a thickness of ~90 μm ( Figure R9a), which agrees with the theoretical value (84.4 μm). The cross-section view of the electrode also shows a relatively smooth surface and uniform distribution of carbon nanofibers (the dark spots belong to carbon nanofibers), indicating an effective confinement of high-loading K and simultaneous expansion of the host and K metal ( Figure R9b).
Consequently, Phase I and II do not affect the intrinsic thickness or volume of the MSCNF host, and the volume change caused by Phase III can be deduced theoretically. We have included Figure R8 and R9 2020,32,1906739), same trend was revealed by Ming and coworkers (ACS Energy Lett. 2020, 5, 3124−3131). This can be ascribed to the highly reactive fluorine bound to the S of sulfone in KFSI, which leads to a fluorine-rich SEI favorable for reversible K plating/stripping. Apart from the anion effect, solvents (for example, Briefly, apart from a highly potassiophilic and robust host, a high CE eligible for the practical application of K metal batteries (>99.99%) would be realized together with a rational electrolyte design that combines following features: (1) suitable anions such as FSI − for F-rich SEI formation;

Figure R9
(2) solvent with low dielectric constant and high LUMO levels; (3) high-concentration electrolytes or localized high-concentration electrolytes (aggregated solvent and anions) that provide a stable K + -solvent structure.
Per request, we have now included the above discussion in the revised manuscript (page 10).

viii) A minor point is that how the specific capacity was calculated should be discussed in main text for clarity. Now it is discussed in SI.
Response: In detail, the specific capacity of the infused MSCNF-K electrode was collected by discharging the MSCNF-K||Cu coin cell at 0.02 mA cm −2 , whose value was determined based on the total mass of K metal and MSCNFs in the composite anode.
For a K-S full cell, the specific capacity is calculated based on the mass of Se and S in the cathode.
Per request, we have included the above information in the revised manuscript (method section).

Reviewer #2 (Remarks to the Author):
Dear Authors, The manuscript is well presented, complete and contains high-quality science that will be of significance to the field of batteries. Response: We are grateful for your support on this work. We want to express the gratitude on the reviewers' time and effort to help improve this manuscript. The listed issues are addressed point-by-point.
-Please, find enclosed the attached file, which contains all the typos check in the manuscript and fix them. Response: Per request, we have checked the typos carefully and corresponding revisions have been given and highlighted in yellow in the revised manuscript (page 2, 3, 4, 5, 6, 7, 8, 9, 10, 14, 19, 22 and 23). Response: Per request, we have changed the description of in-situ XRD into operando XRD in the revised manuscript accordingly (caption of Figure 3; page 11; method section).
For the electrode preparation, MSCNFs were cut into circular pieces of 8 mm diameter and directly utilized as the cathodes for K deposition. For a control experiment, an Al foil was used to act as both a non-porous substrate for K deposition and a window for X-ray transmission.
The operando cell setup is shown in Figure R10.  - Figure 4. Please, add in the caption that the spectra are XPS spectra. It might be not obvious to some young readers.

Response:
Per request, we have now added the description "XPS spectra" into the caption in the revised manuscript ( Figure 5).
-Etching of the MSCNFs. Could, the authors elaborate, please, on the choice of HCl?
And add this explanation in the preparation section.

Response:
As a common method, HCl etching is usually selected to remove acid-sensitive species or templates in certain composites without damaging their major or parent components (Angew. Chem. Int. Ed. 2019, 58, 12469-12475). Here we use HCl solution to react with Zn clusters in order to obtain Zn-free porous carbon nanofibers.
The reaction is based on the following equation: 2HCl + Zn = ZnCl2 + H2.
We have included the above information in the revised manuscript (method section).  As shown in Figure R11, the X-ray diffraction (

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): The authors have properly adressed all raised points in the first report. The manuscript can now be recommended for publication.
Reviewer #2 (Remarks to the Author): Dear Authors, thanks for addressing all my comments and improving the quality of the manuscript. In my opinion, the paper is now ready to be accepted for publication in Nature Comm.
Best regards.